1. Field of the Invention
The field of the present invention relates to earphone devices for acoustic sound reproduction.
2. Background
The advent of MP3 players has led to a greater uptake of earbud style earphones for listening to music. There are two general categories of conventional earbud earphones: (i) those that simply rest in the Concha in proximity to the ear canal, and (ii) those that effect some form of contact with the ear canal to attempt to form an acoustic seal.
The former category have no means of sealing to the ear and suffer from, among other things, poor bass response, very little sound isolation and an insecure fit in the ear.
The latter type, often known as insertion earbuds, form a seal to the ear canal by protruding into the ear canal to a greater or lesser extent and using some form of flexible element to make contact with the ear canal. They offer improved bass response, improved isolation and a more secure fit than proximity type earbuds. In practice however, the efficacy of the seal falls far short of the ideal and leads to a loss in bass performance, isolation, fit and comfort.
In order to reproduce a deep bass response, the earbud should effectively seal to the ear canal so as to minimize air leakage between the earbud speaker and the ear canal. If significant air leakage exists between the earbud and the ear canal, the frequency response at the eardrum will exhibit a falling bass response—the larger the leak, the higher in frequency that the bass response will begin to fall. The seal of an insertion type earbud is generally achieved by some means of flexible cushion that fits to the ear canal or ear canal entrance. The bass response is also affected by the acoustic impedance of this cushion. If the impedance is too low, then the bass response will be lowered.
Attempts to compensate for the bass response problems experienced with insertion type earbuds have generally been unsuccessful. Attempting to compensate for the falling bass response by the use, for example, of a suitable equalizing filter prior to the amplifier driving the earbud can lead to overload problems. The air leak lowers the acoustic impedance into which the earbud drive unit operates; thus, to maintain a given level of bass output, the drive unit has to be driven harder to produce more displacement, and the amplifier must produce a commensurately greater output level. Both these factors result in higher distortion from the earbud and a reduced maximum output level capability. The requirement for increased drive is difficult enough with moving coil drive units, but is particularly problematical with earbuds that make use of miniature balanced armature drive units which typically have a much lower acoustic volume velocity capability at low frequencies.
The quality or characteristics of the seal relative to the ear canal can also affect the acoustic isolation characteristics of the earbud. The better the seal, i.e., the less air leakage, the greater the acoustic isolation, particularly at low frequencies. Insertion type earbuds are often worn in environments where their ability to block external noise is important, thereby allowing listening at safe audio levels in the presence of significant environmental noise, such as on planes, public transport etc. A poor seal will greatly reduce the ability of the earbud to block external noise, particularly at lower frequencies, and any attempt to make the low frequencies more audible by boosting the volume could, for example, lead to damaging playback levels or increased distortion.
In addition to the ability to minimize air leaks, the acoustic impedance of the cushion employed to effect the seal is also important. If the cushion is too light and flimsy, it will have a low acoustic impedance and will act as a poor barrier to external noise, irrespective of its effectiveness in eliminating acoustic air leaks.
In prior art earbuds such as the ER4S® earphone made by Etymotic Research, Inc., the seal has typically been implemented by one of two means. The first is shown in FIG. 1 and comprises a three-tiered ‘mushroom’ tip that is attached to the output port of the earbud housing. A tight seal is maintained between the housing and the tip so that, ideally, no acoustic leak occurs between the two. The mushroom tip is inserted into the ear canal so that one or more of the mushroom caps makes contact with the walls of the ear canal to effect a seal. Due to the differing dimensions of the ear canal from person to person, ear tips are made available in different sizes, and the user must determine by feel and by listening to audio just which is the most appropriate tip. However, in order to effect a suitable seal, the tip has to be inserted quite deeply into the ear canal and can prove uncomfortable so that many users do not in practice get the benefit of a sufficiently complete seal. Also, due to the deep insertion, the tip often picks up ear wax that is not easily accessible to removal when cleaning the ear canal.
There is a further drawback of such earbuds even when a good seal is obtained, which is related to the limited ability of the seal to block external noise. The mushroom tip caps are made of a very thin silicone rubber so that they will deform easily when inserted into the ear canal. However, this flexibility is detrimental to the ability of the cushion to block external noise. Graph 301 in FIG. 3 shows the acoustic attenuation of an example earbud in a real ear using a mushroom cap (as compared to the attenuation of a foam cap, discussed below). As can be seen, the high frequency attenuation levels off above a few hundred Hertz instead of continuing to improve with increasing frequency.
An alternative method is to use a slow recovery polyurethane foam cushion, similar to the earplugs used for protection in noisy environments. Such a scheme is offered with the ER4® earphone as an alternative to the mushroom tip and is shown in FIG. 2. The foam cushion has to be carefully rolled to compress the foam prior to insertion in the ear canal, taking care not to cause creases in the foam that would prevent even expansion once inserted into the ear canal. Once compressed, the foam cushion is pushed into the ear canal, as deep as possible, whereupon it expands to make contact with the ear canal walls and effect an acoustic seal. In order to make an effective seal, the expansion of the foam has to be significant and this leads to a feeling of pressure in the ear canal once fully inserted, which can lead to discomfort. The same problems exist as with the mushroom cap in respect of picking up earwax.
The acoustic impedance of the foam cushion can be significantly higher than that of the mushroom cap however, leading to a greater isolation at high frequencies. This can be seen by again referring to the acoustic attenuation graph shown in FIG. 3, and in particular to graph 305. The lower frequency attenuation is similar for both cushions, indicating that a reasonable low frequency seal has been obtained for both cushions, but the high frequency attenuation of the foam cushion as shown by graph 305 is clearly superior.
A problem alluded to in the above description is the requirement for the wearer to select the correct size cushion and insert it deeply enough and carefully enough to obtain a good seal. Users are reluctant to push the cushion deeply enough into the ear, for fear of causing damage to the ear if inserted to deeply. This problem is common to that found with earplugs, and studies show that users are not very adept at obtaining the best performance from these devices (see Elliott H. Berger, “The Naked Truth About NRRs,” EAR Hearing Protection Products; 1993; reproduced in EARLog by EAR Aearo Company).
A further type of ear seal can be found on earbuds such as the MDR-NC11 noise-canceling earbud headphones made by Sony Electronics Inc. These use a variant of the mushroom tip utilizing just a single cap, again with different sizes to accommodate different ears. This style of earbud generally employs a moving coil transducer having a larger diaphragm than the balanced armature devices, but still requires a good seal for reproduction of the bass frequencies. The tips have the same issues as with the multiple cap approach but are not designed to fit as far into the ear and suffer from the same problems of poor seal and poor isolation, exacerbated by the shallow insertion.
It would therefore be advantageous to provide an earphone device that overcomes one or more of the aforementioned problems, disadvantages or drawbacks. It would further be advantageous to provide an earphone device such as an earbud fitting into the ear canal that generally requires less skill or attention from the wearer whilst potentially providing other additional benefits including, for example, an improved seal, a higher acoustic impedance cushion and/or a more secure fit in the ear to improve the bass response of the earbud. It would also be advantageous to provide an earphone device that provides more effective noise isolation and improved consistency of fit and performance.